This invention relates to electrical circuit devices comprising porcelain enameled coated metal substrates such as printed circuit boards wherein conductive patterns are applied to the surfaces of the porcelain enameled coated metal substrates. In particular, the invention relates to forming high strength solder connections between a flexible conductor, for example, a wire or the lead of an electronic component and the printed circuit boards formed of porcelain enamel coated metal substrates. The invention also relates to the devices formed in accordance with the methods of the invention.
Many modem printed circuit cards comprise substrates formed of porcelain enameled metal. Circuits are formed on the substrates using conductive, dielectric or other resistive materials.
The resulting coated metal substrate can be used for circuit boards, thermal sinks, thermal barriers, RF shielding, magnetic flux conduction, mechanical attachments and other related applications. Such coated metal substrates that are coated with a porcelain enameled material are commonly referred to as porcelain enameled metal substrates (PEMS).
Discrete circuit patterns can be formed on the PEMS by conventional techniques such as screen printing, firing, etc. The circuit patterns can serve as switch contacts, circuit traces to carry current, and terminals for interconnection of external wires. The circuit patterns may be formed on one or both sides of the substrate.
In the prior art, in order to attach a wire or other conductor such as a lead to the circuit formed on the PEMS, an aperture would be created in the substrate, the wire or conductor would then be inserted into the aperture, and the wire or conductor would be soldered to the electrical circuit. Alternatively, the wire or lead was soldered to a pad formed by thick film material on the surface of the substrate. However, these conventional soldering techniques resulted in weak joints. The weakness of the solder joint at the connection is a very common problem. Typically the strength of the solder joint on these electrical circuit devices are less than half the pull strength of similar solder joints on electrical circuit devices comprised of, for example, alumina ceramic. Thus, a need exists for a method to secure conductors to a PEMS that provides high mechanical strength and durability as well as good electrical conductivity.
In view of the aforementioned needs, the invention contemplates a durable, high strength interface or connection between a printed circuit board comprised of porcelain enamel coated metal substrates and an external conductor such as a wire or a lead of an external electronic component.
In the method of the present invention, an aperture is first created in the substrate. The aperture is formed in a manner to ensure that the metal substrate is electrically insulated from the aperture. Then, an eyelet is inserted into the aperture. The eyelet is usually crimped, but other means are available that will cause the eyelet to be mechanically retained in the aperture and form an electrical connection between the eyelet and circuitry formed on the surface of the printed circuit board. A conductor is then inserted into the eyelet and then soldered to the eyelet such that the thick film adjoins the eyelet. The addition of an eyelet to the soldered joint connection mechanically reinforces the solder joint and provides unexpected vastly improved mechanical strength to the joint.
Among those benefits and improvements that have been disclosed, other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.